Recovery of oil from olives is a major industry. One process for oil recovery which reaches into antiquity is simple pressing. High oil content seeds, such as sesame and peanut, and the oily pulp of olives yield free oil by the simple application of pressure. Oils of this type require no further processing. However, simple pressing is not efficient and is not utilized for processing underripe or green olives.
The olive meat remaining from simple pressing contains a substantial amount of valuable oil. This led to the development of more efficient presses, such as the hydraulic batch press and the expeller. These presses operate on a batch or noncontinuous basis and typically develop pressures of about 1 to 15 tons per square inch, leaving 2 to 4% oil in the meat. Unfortunately, such presses also develop excessive heat. This causes darkening of the oil and denaturization of olive oil protein.
Extracting oils on a batch basis is labor intensive. A high degree of operator attention is required to monitor pressure, the progress of oil extraction and other factors. Sophisticated machinery can be used to avoid human error; however this significantly increases capital and maintenance costs.
The post-press solvent extraction method was introduced to reduce damage to olive oil during crushing and as a means of removing all of the available oil. In this process, the olive is batch pressed to remove only part of the available oil. The temperature at this point is not sufficient to degrade the oil or protein to any great degree. Solvent extraction, typically performed with hexane, is then used to remove the balance of oil from the meat.
However, solvent extraction requires careful control and removal of the solvent, which has the potential of being carcinogenic. Olive oil which has been treated by any method using a solvent cannot be marketed as "virgin" olive oil.
The conventional cold-pressing process of olive oil extraction involves several steps. First, ripe olives are picked and stored awaiting extraction. Good practice includes placing these stored olives under a nitrogen blanket to prevent spoilage.
The next step involves crushing which is performed in a press on a batch basis. The whole olives are first shredded and squeezed until the pits are broken. These broken pits function as a conduit to allow the liquid to exit.
The length of liquid travel, from the center of the pressed material to an exit port, can be as great as one foot in a typical two-foot diameter batch press. Such a long liquid travel length reduces the efficiency of liquid extraction to an extent that multiple pressings are sometimes required. However, a reduced press diameter would not solve the problem because it would require a greater number of pressings to process an equivalent volume of olives and correspondingly greater operator attention.
The liquid (water and oil) runs about 30% pulp, which must be removed before the liquid can be purified. The liquid is allowed to settle and the pulp is removed by decantation. The clarified liquid is centrifuged to separate the water from the oil so that the oil has a water content of less than about 1%.
Depending on the quality of the olives used, which depends on the percentage of rotten olives processed, the oil may then be decolorized by making a soapstock to remove any free-fatty acids present, or by filtering with activated carbon, or both.
The oil recovered in the cold-pressing conventional process contains an oil from the olive pit which has a bitter taste. This inferior oil corrupts the product olive oil and lowers its quality.
The pulverized olive meat contains nutrient values and has potential salvage value. However, the sharp broken pit pieces are mixed thoroughly with the meat and present a safety hazard for animals, if the meat is to be used as a feed material. The pits are not removed before crushing because the pieces are needed during the squeezing process to provide a conduit to allow the liquid to exit. To then separate the broken pits from the pulverized meat would be an expensive and inefficient proposition.
There is need for a process for olive oil recovery which yields an olive oil that is pure, contains no solvents, has not been denatured, and which is free from the bitter taste of the olive pit. There is also need for an olive oil recovery process which efficiently salvages the pulverized olive meat and pits, and which does not require a great degree of operator attention.